Embodiments presented herein relate generally to ultrasound imaging systems, and more particularly to synthetic transmit focusing of ultrasound images.
Ultrasound imaging is widely used for non-invasive imaging of interior portions of the human body. Medical ultrasound imaging, also known as ultrasonography, is a diagnostic medical imaging technique for visualizing the cardiovascular system and other internal organs and structures with real time images, both two-dimensional (planar) and three-dimensional (volumetric). When used to image the cardiovascular system, heart mechanics, valve motion and blood flow can be visualized in real time. When used to visualize major organs, their size, structure and any pathological lesions can be observed. Ultrasound is also used to form volumetric images of fetuses during routine and emergency prenatal care.
Ultrasound imaging systems may use a transmit array to transmit ultrasound pulses, a receive array to receive echoes of the transmitted ultrasound pulses, and an image reconstruction system to translate the received echoes into images. In most systems, the transmit and receive arrays are the same physical device, and a means exists for switching the device between a transmit mode of operation and a receive mode of operation. An image is typically formed by scanning a beam. That is, beams of ultrasound pulses are formed in a sequence of beamsteering angles with respect to the array normal and a focused transmission and reception occurs in each such direction. The resulting sequence of one-dimensional data sets represents reflectivity data as a function of range in each of the beam directions. This measured acoustic echo data set covers a two-dimensional patch of the reflectivity distribution in front of the array and can be converted into a displayable image.
The standard approach to image reconstruction using such a data set typically involves scan conversion, a process by which samples of demodulated, formed beams are interpolated from a range/angle grid into a rectilinear one. An alternative scheme is the direct reconstruction of image samples in an arbitrary output arrangement that could be rectilinear. Such direct reconstruction may be better for software image formation on massively parallel processors, while the standard method may be better suited to special purpose hardware. The basic operation of forming a single sample is the same for both approaches, although it may typically be implemented differently.
The image feature corresponding to a point reflector is called the point spread function (PSF) of the system. The size of the PSF is often used as an indicator of the potential image quality of a system. The dimensions of the PSF of an ultrasound system are different in two orthogonal directions: the axial response along the direction of sound propagation is determined by the duration of the ultrasound pulse, and the lateral response transverse to the propagation direction is determined by the transmit and receive focal properties at the depth of the point of interest. At any point in the image, the PSF is the product of a transmit point response with a receive point response. The receive lateral point response can be dynamically adjusted, during reception of the acoustic data or at the time of image reconstruction so that every point in the image is in good receive focus. Such a dynamic adjustment is standard practice in modern ultrasound imaging equipment. In a scan-converter-based system, dynamic receive focus is typically done during the reception of the acoustic data. In a direct reconstruction system, array element data may be recorded which allows dynamic receive focus to be performed for every reconstructed output sample. In either case, reconstructed samples are all individually receive focused. The transmit lateral point response depends on the aperture size and the delay structure used to form the transmit beam.
Most ultrasound imaging systems focus the transmit array to a fixed depth of interest, in order to maximize lateral resolution at that depth. The echoes received from a depth corresponding to the transmit focus can form relatively sharp images, while those received from other depths will produce more lateral spread in the PSF. Just as in an optical lens system, the more highly focused a transmit beam is, the smaller the depth of field over which the tightest focus is observed. Thus, the resolution of the image based on focused ultrasound transmit beams is not uniform and degrades as a function of the difference between the depth of the point of interest and the focal depth.
Some known systems reconstruct an image using multiple transmit focal zones to form an image with uniformly high resolution at all depths. Such a procedure requires the transmission of multiple ultrasound beams for each beamsteering direction, which causes delays in the generation of a complete image and consequentially reduces the frame rate.
For some years, there has been interest in the concept of reconstructing focused images without using the multiple focal zone approach. Such a technique would allow the formation of a highly focused image at higher frame rates than are currently available. In general, such techniques are known as synthetic transmit focus imaging. Typically, the image reconstruction system may coherently superpose the receive data obtained from multiple transmit beams, each beam corresponding to different beamsteering and/or transmit focus characteristics, to synthesize images that are in focus at all ranges. Schemes of this kind can be implemented in both standard image formation systems and direct reconstruction systems.
The most basic synthetic focusing technique is called by a variety of names, including “N-squared synthesis”, “direct synthesis” or just “synthetic focusing”, and consists of transmitting from each of a set of transmit elements and receiving the echoes from each transmission at a set of receive elements. In a typical monostatic (reflection mode) case, both sets of elements correspond to the elements of a single transmit/receive array. The N-squared approach has been called the “gold standard” because every reconstructed point is in full transmit/receive focus and, in principle, no image formed with the same array can have a better focus. More recently, this approach has been called the “total focus method” (“TFM”).
The earliest synthetic focus acoustical imaging procedure was called the synthetic aperture focusing technique (SAFT). SAFT consists of scanning a single transmit/receive element over the aperture plane and recording the received waveform at every transmit position. If the transmit/receive aperture is an array, this corresponds to transmitting with each element and recording the echoed data only at the transmit element, thus, SAFT measurements are a subset of TFM measurements.
In general, SAFT and TFM both require many transmits to form an image, and this can present problems when the reflectivity distribution is in motion, since the measurements have to be coherently combined. Additionally, single element transmits typically result in poor penetration. A compromise method that uses fewer, higher-intensity transmits is that of coherent compounding of transmit wavefronts from focused and unfocused beams. In these schemes a multi-element subarray is used to insonify the reflectivity distribution, and the data from multiple transmits, possibly in different beamsteering directions, is coherently compounded by, for example, adding complex analytic signal values derived from multiple dynamic receive beamformers focused at a single point in space. In general, these schemes are approximations to TFM.
All of these known synthetic focus techniques make use of data sets that are unique to them, rather than using the scanned beam data set employed in standard ultrasound imaging. In the recent past, there has been interest in the concept of reconstructing focused images using unfocused portions of transmitted ultrasound beams in the standard, scanned-beam data set. This approach is called retrospective transmit focus (RTF) as opposed to synthetic transmit focus. Naturally, RTF is attractive to manufacturers with an existing base of ultrasound imaging equipment, since the transmit beamforming subsystem of an existing instrument does not have to be modified in order to migrate that instrument to RTF operation.
One such scheme uses the converging portions of focused beams to perform a synthesis. Another coherent compounding approach is based on planewave insonification. In general, since such prior art techniques are based on spatial compounding of unfocused beams, the resulting synthetic PSF can have reduced contrast due to high sidelobes, which are in turn due to the lack of transmit focus of the component transmit beams. This is because portions of propagating wavefronts not used for mainlobe synthesis contribute very energetic transmit sidelobes that can extend far from the synthesized PSF mainlobe, in some cases.
Some known techniques use a spatial or spatial/temporal matched filter on every element of the receive array, prior to receive beamforming to reduce the effective transmit response away from the point to be reconstructed. The associated synthetic transmit focus schemes have used SAFT, so these are not RTF approaches. The filter is matched to the expected reflection from a point to be imaged. Such a technique is known to result in a slight loss of axial resolution.
The propagation of ultrasound in fluids can be highly nonlinear, and this nonlinearity results in high-amplitude propagating ultrasound waves being converted from purely harmonic waves to sawtooth waves. This change of shape results in a transfer of energy from the fundamental frequency region of the energy spectrum to the regions near the harmonic frequencies. A standard mode of operation in modern imaging instruments is to select the second (or higher) harmonic portion of the received signal by filtering or other means and use the selected portion of the reflection signal for imaging. This mode of operation is called tissue harmonic imaging (THI). It would be advantageous for any RTF scheme to operate in THI mode as well as in fundamental imaging mode.
Therefore, there is a need for an ultrasound imaging system that produces highly focused images with the high contrast. There is also a need for a system that operates from low PSF sidelobes at all depths, at high frame rates, using standard arrays and transmission schemes, and operating in both fundamental and THI modes.